Anisotropic conductive connection can be required to electrically connect connection electrodes of an IC circuit board to terminals of a substrate mounted on a circuit board, such as a liquid crystal display (LCD) panel. Film-type adhesives are widely used as anisotropic conductive packaging materials, in which conductive particles, such as metal-coated resin particles or metal particles, are dispersed in an insulating resin, e.g., epoxy, urethane, or acrylic resin.
Conductive particles are interposed between electrodes and terminals by disposing the anisotropic conductive packaging material containing the conductive particles between the electrodes and the terminals and applying pressure and heat to adhere the packaging material therebetween. Currently, electrical connection occurs in the pressing direction, and an insulation state is maintained in a direction perpendicular to the pressing direction due to the presence of insulating components contained in an insulating adhesive.
In LCD packaging requiring anisotropic conductive connection, recent advances in LCD technologies have increased connection pitch compactness, IC bump minuteness and the number of leads printed on substrates. Further, there continues to be a need for improved electrical connection reliability. In order to satisfy such technical needs, conductive particles contained in anisotropic conductive films are largely required to have a uniform and small particle diameter. Further, it can be critical that conductive particles have enhanced conducting properties without being ruptured, together with appropriate compressive deformability and recoverability from deformation, because of increased contact area with connection substrates when the conductive particles are interposed and compressed between the connection substrates. Metal particles, such as nickel, gold and silver particles, and metal-coated base particles can be used as the conductive particles. However, since metal particles have a non-uniform shape and a much higher specific gravity than an adhesive resin, they can have poor dispersibility in the adhesive resin.
For these reasons, in mounting applications requiring superior connection of microelectrodes and improved connection reliability, conductive particles with a uniform shape, a relatively narrow particle diameter distribution and enhanced conducting properties are widely used as a plated layer formed on base polymer particles.
Extensive research has hitherto been conducted on conductive particles in which polymer particles are plated, and particularly on the characteristics of the particles after compressive deformation in terms of improved contact with electrodes and connection reliability.
For example, Japanese Patent Laid-open No. S63-107188 discloses the use of high-strength highly elastic conductive particles with a compressive strength of 500 kg/cm2 and a high compressive elastic modulus of 80×103 kg/cm2 or more. Further, PCT Publication WO 92/06402 discloses a spacer for a LCD and conductive particles using monodisperse resin particles as base particles. According to this publication, in order to readily control a gap between electrodes facing each other when the electrodes are connected to each other by compression using the conductive particles, the resin particles preferably have a compression hardness at 10% compressive deformation (10% K value) of 250 to 700 kgf/mm2. In addition, in order to increase the contact area between the conductive particles and the electrodes after compression, the resin particles preferably have a recovery factor after compressive deformation of 30 to 80%.
Further, Japanese Patent Laid-open No. H07-256231 discloses conductive particles having a K value at 10% compressive deformation of 700 to 1,000 kgf/mm2 and a recovery factor after compressive deformation of 65% to 95% at 20° C. in order to improve poor conductivity caused by changes in the temperature between electrodes, folding, mechanical impact, and the like.
Moreover, Japanese Patent Laid-open No. Hl 1-125953 and No. 2003-313304 disclose conductive particles having a K value at 10% compressive deformation of 250 kgf/mm2 or lower and a recovery factor after compressive deformation of 30% or greater for better connection reliability.
These patent publications note that as the recovery factor after compressive deformation of the conductive particles increases over a broad range of hardness, the conducting properties of the conductive particles, e.g., increased contact area with the electrodes, are enhanced.
However, when such conductive particles are dispersed in a cure-type binder resin and are pressed under heating to connect to electrodes, the adhesive force of the curing binder resin and the contact of the particles with the electrodes are often insufficient. Recently, fast curing processes of anisotropic conductive adhesive films at low temperatures within a short period of time have been increasingly employed. These short-term connection conditions enable rapid curing of a binder resin, but cause insufficient contact of conductive particles with electrodes.
In addition, in anisotropic conductive materials, e.g., connection films for chip on glass (COG), having a greatly increased content (by several tens of %) of conductive particles, a large amount of highly elastic conductive particles interposed between electrodes partly or wholly deteriorate the adhesion of the connection films, resulting in poor connection reliability over a long period of time.
In addition to minuteness of electrode patterns and compactness of connection pitches, low pressure processes are required to connect relatively weak wiring patterns, e.g., ITO electrodes to avoid damaging the wiring patterns. However, low pressure connection conditions can markedly deteriorate the adhesive force and connection reliability of highly elastic conductive particles.